1. Field of the Invention
The present invention relates to an image display apparatus synthesizing a plurality of laser beams of different color components to project a color image and an optical component utilized in such an image display apparatus, exhibiting a prescribed reflectance or transmittance for a laser beam of a prescribed wavelength.
2. Description of the Background Art
Various image display apparatuses, such as a laser projector, each synthesizing laser beams of a red color component (R), a green color component (G), and a blue color component (B) to project and display a color image on a projection surface are put to practical use. Such an image display apparatus must maintain the color state of a beam obtained by synthesizing laser beams of R, G, and B in a prescribed state in order to project and display a color image with high color reproducibility. Typically, the white balance of the beam obtained by synthesizing the laser beams of R, G, and B must be created.
Japanese Patent Laying-Open No. 2004-226631 discloses a projector including a first light source portion, a second light source portion, a third light source portion, a temperature detection portion detecting the temperature of each light source portion, a spatial light modulator modulating light from each light source portion according to an image signal, a projection lens projecting the light modulated by the spatial light modulator on a screen, a storage portion storing a relationship between the temperature of each light source portion and a luminance distribution of each color light from each light source portion, and a control portion controlling the spatial light modulator to substantially uniform the luminance distributions of the color light on the screen on the basis of the relationship between the temperature of each light source portion detected by each temperature detection portion and the luminance distribution of each color light stored in the storage portion in order to reduce irregular colors correspondingly to a change of irregular color distribution following a change of the temperature of each light source portion.
Japanese Patent Laying-Open No. 2008-003270 discloses the structure of a projection image display apparatus partially emitting a beam combined into a white beam to a light detection portion, allowing the beam to be incident on a photo sensor having a color filter mounted on a photodiode, comparing a signal indicating the light quantity of the beam output from the photo sensor with a signal value in previous adjustment of a white balance by changes of the light quantities of R, G, and B in a control portion, and changing the drive gain of each light source in order to reduce a difference between the light quantities to adjust the chromaticity of white.
Japanese Patent No. 2663437 discloses a semiconductor laser apparatus configured to reduce a fluctuation in the intensity of output light resulting from a temperature change by rendering the temperature-dependent characteristics of the intensity of monitor light emitted from a semiconductor laser element and the temperature-dependent characteristics of the detection output of a light receiving element opposite to each other. In this semiconductor laser apparatus, no control for achieving a prescribed color state such as a white balance correspondingly to a wavelength change is performed.
As described above, in the image display apparatus projecting the color image, it is necessary to create the white balance, but a change of the output light quantity of each light source and a change of the output wavelength of each light source cause loss of this white balance. In other words, a state where the white balance is created denotes a state where the light quantity (namely, the output of each light source) thereof is a prescribed value with respect to the wavelength of the beam of each color component forming a synthesized beam, and when the wavelength of the output of each light source changes, the white balance is lost unless each light source outputs a prescribed light quantity with the changed wavelength.
In the image display apparatus, an element such as an LD (a laser diode) is employed as a laser beam source for each color component, but the wavelength of an output laser beam changes due to a temperature change. On the other hand, the image display apparatus is employed in various environments and is provided with a component generating heat by operation in addition to the laser beam source, so that the temperature of the laser beam source changes and the wavelength of the laser beam output therefrom changes. In the case where the white balance is adjusted on the basis of the light quantity (the luminance) of the laser beam, the white balance of a synthesized laser beam is disadvantageously lost when the wavelength of the laser beam changes.